The continuing popularity and growth of still and motion photography among professionals and amateurs has led to the development of a number of automated features, probably the most significant of which is automatic focussing. While there are severally accepted autofocus techniques, contrast autofocus is one of the more commonly used, particularly in many video cameras and camcorders. In a contrast autofocus system a motor driven displacement mechanism axially translates the camera lens to achieve a precise focussing position in response to a comparison of successive through-the-lens images on a solid state imager at an equivalent film plane. Specifically, this technique compares repetitive image samples which are taken as the lens position is changed. When the camera is not focussed on any subject, the sample image is blurred and therefore low in contrast. Displacing the lens to bring the subject into focus reduces the blurring and thus increases the contrast in the image.
The problem with conventional contrast autofocus schemes is that they usually require many measurements of contrast, so that the lens will be driven until it is displaced past optimum focus whereupon the contrast begins to degrade; the lens is then driven back in the opposite direction towards the peak contrast (sharpest contrast) point. This inability to continually and rapidly displace the lens to optimum focus is due to the fact that conventional contrast autofocus schemes lack a mechanism for determining, at the outset, in which direction to move the lens to attain focus and how far away the current lens position is from achieving focus. Although other autofocus techniques, correlation autofocus in particular, overcome some of these problems, they require additional customized optics in order to create two, split images, plus twice the light level to successfully operate, conditions that are not always acceptable.